The invention relates to a method and apparatus for checking a sensor. Specifically, the present invention relates to a method of checking a sensor which generates an electrical measuring signal on the basis of a non-electrical physical value.
A known method of checking sensors consists of applying check signals or an electrical check voltage to the sensor and then studying the pattern of the check signal or the sensor output signal generated during the check procedure to determine whether it is within predetermined limits. The check signal and the physical parameter measured by the sensor bear no direct relationship to one another so that normally only errors such as short circuits, line breaks, or the like can be detected. The check signal and the measured parameter, however, do not determine whether the sensor is operating reliably over its entire measurement range. Since a check signal is applied to the electrical measuring circuit, occasionally the test signal generated by application of the check signal (sensor output signal) cannot be distinguished from a useful signal or a defective useful signals.
It is also known that a sensor can be checked by interrogating the sensor during known states of the object being measured (or during the shutdown of a component whose rotational speed is to be measured), and comparing its output signal with the known value. Similarly, a reference object with precisely known properties is measured by the sensor for calibrating or gauging a sensor. In both cases the sensor can only be checked during precisely known states of the measured object or a comparison standard, but not during normal use of the measured object (for example, if the sensor is installed in a device).
It is an object of the present invention to provide a method and apparatus for checking a sensor which permits reliable and unambiguous checking of the sensor over its entire measurement range.
This and other objects and advantages are achieved by the apparatus according to the present invention, in which a sensor, via a test value transmitter, is actively subjected to the physical parameter to be measured. The sensor and the test value transmitter therefore operate according to the same principle. The sensor and the test value transmitter also generate an electrical signal under the influence of a non-electrical physical parameter and conversely generates the same non-electrical physical parameter when an electrical signal is applied. Non-electrical parameters are generally considered to be magnetic, mechanical, thermal, optical, and chemical parameters.
The test value transmitter uses an electrical check signal to generate the non-electrical physical parameter that acts on the checked sensor. The sensor then generates an electrical output signal referred to hereinbelow as the check signal. The check signal that excites the test value transmitter, along with the test signal generated during a checking procedure by the sensor, bear a direct correlation to one another and depend linearly upon one another in converters with linear characteristics. The proportionality factor of the linear dependence depends on the efficiency and degree of coupling between the sensor and the test value transmitter with regard to the non-electrical physical parameter and is used as the known constant in the above configuration. If the check signal and test signal correspond in light of this factor, this is a reliable indication that the sensor is functioning reliably, since the sensor is checked using its own measuring principle.
In an embodiment of the invention, the sensor and the test value transmitter are integrated into one structural unit, so that the sensor (even if it is built into a device), can be checked at any desired time. It is also especially advantageous that the electrical measuring circuit of the sensor and the electrical check circuit of the test value transmitter can be completely separate from one another, so that these circuits operate without any feedback. For example, an electrical short in the sensor has no effect on the electrical check circuit and vice versa, so that the danger that would otherwise exist is eliminated (namely that a defective electrical circuit could destroy structural elements in the other electrical circuits).
In preferred embodiments of the invention, the sensor and the test value transmitter are electromagnetic converters (coupled electromagnetically with one another), induction coils, Hall probes, piezoelectric transducers (forcibly coupled with one another) or thermocouples (coupled thermally with one another).
In the case of the electromagnetic and piezoelectric converters, according to an object of the invention, it is also possible to superimpose the testing procedure on the actual measurement procedure of the sensor, (i.e., to check the sensor during its measuring operation). This can be accomplished, for example, if the physical parameter to be measured and the physical parameter generated by the test value transmitter differ in frequency so that the output signal from the sensor contains the actual measurement or useful signal and the test signal. These two signals are separable from one another by frequency filtration.
Another important advantage of the invention is that the sensor can be checked over its entire measurement range with the check signal, and hence the physical parameter generated by the test value transmitter can scan the entire measurement range of a sensor during testing.
The invention will now be described in greater detail with reference to the drawings.